In the field of electronic components, the bonding of copper, a metal having very good electroconductive properties, to aluminum oxide ceramic, an electrical insulator but good thermal conductor, is of particular technical significance. The copper layer finds utility as a conducting track layer, electrode layer, sensor layer, or as a bonding layer when the aluminum oxide ceramic is soldered as a heat sink to other, heat-evolving devices.
There are basically two different processes in existence for directly applying copper to the ceramic material. DE-A-3 824 249 describes a process comprising electroless deposition following prior roughening of the ceramic surface and a subsequent palladium nucleation followed by heat treatment, while DE-A-3 036 128 relates to a direct copper bonding (DCB) process wherein copper plating from 100 to 900 .mu.m in thickness is applied to the ceramic surface at about 1070.degree. C. Both processes are technically complicated and, in addition, are not suitable for producing copper structures of a very precise geometry because of the necessary roughening of the ceramic or because of the thickness of the copper layer. Moreover, the ceramic-copper bonds produced by the existing processes are of only very limited usefulness in high frequency technology.
For this reason a frequently adopted approach has been to apply the basic metal layer by the method of cathode sputtering in a vacuum, hereinafter simply referred to as sputtering.
However, it has been known for a long time that copper applied by this technique to ceramic shows only very poor adhesion. For this reason it has been common for some years to interpose between the ceramic and the copper a thin layer, a few 10 to 100 nm in thickness, of a coupling agent comprising a reactive metal applied by sputtering. The usual coupling agents used are metals such as chromium, titanium, nickel, tantalum, tungsten, and alloys such as TiW or NiCr. Although these metallic coupling agents result in an excellent adhesion of 50 MPa or more, this technique has two crucial disadvantages. First, the sputter coater must contain two separately controllable targets, namely the coupling agent and copper. Secondly, the later structuring of the metal layer requires an additional etching step, since none of the coupling agents is structurable with an etching medium suitable for copper. There is yet a further disadvantage in that the etching media for the coupling agents are usually very aggressive and hence very difficult to handle, particularly on a industrial scale.